Semiconductor devices are used in a variety of electronic applications, such as personal computers and cellular phones, for example. The semiconductor industry in general is being driven to decrease the size of semiconductor devices located on integrated circuits. Miniaturization is generally needed to accommodate the increasing density of circuits necessary for today's semiconductor products.
One semiconductor product widely used in electronic systems for storing data is a semiconductor memory device, and one common type of semiconductor memory device is a dynamic random access memory (DRAM). A DRAM typically includes millions or billions of individual DRAM cells arranged in an array, with each cell storing one bit of data. A DRAM memory cell typically includes an access field effect transistor (FET) and a storage capacitor. The access FET allows the transfer of data charges to and from the storage capacitor during reading and writing operations. In addition, the data charges on the storage capacitor are periodically refreshed during a refresh operation.
DRAM storage capacitors are often formed by etching deep trenches in a substrate. A plurality of layers of conductive and insulating materials are deposited in order to produce a storage capacitor that is adapted to store a bit of data, represented by a one or zero. Prior art DRAM designs typically comprise an access FET disposed in a subsequent layer to the side of the storage capacitor. Some DRAM designs involve disposing the access FET directly above the storage capacitor in the upper part of the trench, which conserves surface area, resulting in the ability to place more DRAM cells on a single chip.
A recent DRAM design comprises an embedded DRAM, wherein logic circuitry and memory cells are fabricated on a single chip. This introduces manufacturing challenges, often requiring that the conductive lines for the memory cells comprise silicon rather than metal, for example. Because silicon has a lower conductivity than metals, often, means for improving the conductivity of silicon are utilized, to improve device performance and increase speed.